rfc2080.txt
来自「RFC 的详细文档！」· 文本 代码 · 共 1,067 行 · 第 1/4 页
TXT
1,067 行
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ... ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Route Table Entry N (20) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Malkin & Minnear Standards Track [Page 5]
RFC 2080 RIPng for IPv6 January 1997
where each Route Table Entry (RTE) has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ IPv6 prefix (16) ~
| |
+---------------------------------------------------------------+
| route tag (2) | prefix len (1)| metric (1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The maximum number of RTEs is defined below.
Field sizes are given in octets. Unless otherwise specified, fields
contain binary integers, in network byte order, with the most-
significant octet first (big-endian). Each tick mark represents one
bit.
Every message contains a RIPng header which consists of a command and
a version number. This document describes version 1 of the protocol
(see section 2.4). The command field is used to specify the purpose
of this message. The commands implemented in version 1 are:
1 - request A request for the responding system to send all or
part of its routing table.
2 - response A message containing all or part of the sender's
routing table. This message may be sent in response
to a request, or it may be an unsolicited routing
update generated by the sender.
For each of these message types, the remainder of the datagram
contains a list of RTEs. Each RTE in this list contains a
destination prefix, the number of significant bits in the prefix, and
the cost to reach that destination (metric).
The destination prefix is the usual 128-bit, IPv6 address prefix
stored as 16 octets in network byte order.
The route tag field is an attribute assigned to a route which must be
preserved and readvertised with a route. The intended use of the
route tag is to provide a method of separating "internal" RIPng
routes (routes for networks within the RIPng routing domain) from
"external" RIPng routes, which may have been imported from an EGP or
another IGP.
Malkin & Minnear Standards Track [Page 6]
RFC 2080 RIPng for IPv6 January 1997
Routers supporting protocols other than RIPng should be configurable
to allow the route tag to be configured for routes imported from
different sources. For example, routes imported from an EGP should
be able to have their route tag either set to an arbitrary value, or
at least to the number of the Autonomous System from which the routes
were learned.
Other uses of the route tag are valid, as long as all routers in the
RIPng domain use it consistently.
The prefix length field is the length in bits of the significant part
of the prefix (a value between 0 and 128 inclusive) starting from the
left of the prefix.
The metric field contains a value between 1 and 15 inclusive,
specifying the current metric for the destination; or the value 16
(infinity), which indicates that the destination is not reachable.
The maximum datagram size is limited by the MTU of the medium over
which the protocol is being used. Since an unsolicited RIPng update
is never propagated across a router, there is no danger of an MTU
mismatch. The determination of the number of RTEs which may be put
into a given message is a function of the medium's MTU, the number of
octets of header information preceeding the RIPng message, the size
of the RIPng header, and the size of an RTE. The formula is:
+- -+
| MTU - sizeof(IPv6_hdrs) - UDP_hdrlen - RIPng_hdrlen |
#RTEs = INT | --------------------------------------------------- |
| RTE_size |
+- -+
2.1.1 Next Hop
RIPng provides the ability to specify the immediate next hop IPv6
address to which packets to a destination specified by a route table
entry (RTE) should be forwarded in much the same way as RIP-2 [2].
In RIP-2, each route table entry has a next hop field. Including a
next hop field for each RTE in RIPng would nearly double the size of
the RTE. Therefore, in RIPng, the next hop is specified by a special
RTE and applies to all of the address RTEs following the next hop RTE
until the end of the message or until another next hop RTE is
encountered.
A next hop RTE is identified by a value of 0xFF in the metric field
of an RTE. The prefix field specifies the IPv6 address of the next
hop. The route tag and prefix length in the next hop RTE must be set
to zero on sending and ignored on receiption.
Malkin & Minnear Standards Track [Page 7]
RFC 2080 RIPng for IPv6 January 1997
The next hop Route Table Entry (RTE) has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ IPv6 next hop address (16) ~
| |
+---------------------------------------------------------------+
| must be zero (2) |must be zero(1)| 0xFF |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Specifying a value of 0:0:0:0:0:0:0:0 in the prefix field of a next
hop RTE indicates that the next hop address should be the originator
of the RIPng advertisement. An address specified as a next hop must
be a link-local address.
The purpose of the next hop RTE is to eliminate packets being routed
through extra hops in the system. It is particularly useful when
RIPng is not being run on all of the routers on a network. Note that
next hop RTE is "advisory". That is, if the provided information is
ignored, a possibly sub-optimal, but absolutely valid, route may be
taken. If the received next hop address is not a link-local address,
it should be treated as 0:0:0:0:0:0:0:0.
2.2 Addressing Considerations
The distinction between network, subnet and host routes does not need
to be made for RIPng because an IPv6 address prefix is unambiguous.
Any prefix with a prefix length of zero is used to designate a
default route. It is suggested that the prefix 0:0:0:0:0:0:0:0 be
used when specifying the default route, though the prefix is
essentially ignored. A default route is used when it is not
convenient to list every possible network in the RIPng updates, and
when one or more routers in the system are prepared to handle traffic
to the networks that are not explicitly listed. These "default
routers" use the default route as a path for all datagrams for which
they have no explicit route. The decision as to how a router becomes
a default router (i.e., how a default route entry is created) is left
to the implementor. In general, the system administrator will be
provided with a way to specify which routers should create and
advertise default route entries. If this mechanism is used, the
implementation should allow the network administrator to choose the
metric associated with the default route advertisement. This will
make it possible to establish a precedence amoung multiple default
routers. The default route entries are handled by RIPng in exactly
the same manner as any other destination prefix. System
Malkin & Minnear Standards Track [Page 8]
RFC 2080 RIPng for IPv6 January 1997
administrators should take care to make sure that default routes do
not propagate further than is intended. Generally, each AS has its
own preferred default router. Therefore, default routes should
generally not leave the boundary of an AS. The mechanisms for
enforcing this restriction are not specified in this document.
2.3 Timers
This section describes all events that are triggered by timers.
Every 30 seconds, the RIPng process is awakened to send an
unsolicited Response message, containing the complete routing table
(see section 2.6 on Split Horizon), to every neighboring router.
When there are many routers on a single network, there is a tendency
for them to synchronize with each other such that they all issue
updates at the same time. This can happen whenever the 30 second
timer is affected by the processing load on the system. It is
undesirable for the update messages to become synchronized, since it
can lead to unnecessary collisions on broadcast networks (see [13]
for more details). Therefore, implementations are required to take
one of two precautions:
- The 30-second updates are triggered by a clock whose rate is not
affected by system load or the time required to service the
previous update timer.
- The 30-second timer is offset by a small random time (+/- 0 to 15
seconds) each time it is set. The offset is derived from: 0.5 *
the update period (i.e. 30).
There are two timers associated with each route, a "timeout" and a
"garbage-collection time." Upon expiration of the timeout, the route
is no longer valid; however, it is retained in the routing table for
a short time so that neighbors can be notified that the route has
been dropped. Upon expiration of the garbage-collection timer, the
route is finally removed from the routing table.
The timeout is initialized when a route is established, and any time
an update message is received for the route. If 180 seconds elapse
from the last time the timeout was initialized, the route is
considered to have expired, and the deletion process described below
begins for that route.
Malkin & Minnear Standards Track [Page 9]
RFC 2080 RIPng for IPv6 January 1997
Deletions can occur for one of two reasons: the timeout expires, or
the metric is set to 16 because of an update received from the
current router (see section 2.4.2 for a discussion of processing
updates from other routers). In either case, the following events
happen:
- The garbage-collection timer is set for 120 seconds.
- The metric for the route is set to 16 (infinity). This causes the
route to be removed from service.
- The route change flag is to indicate that this entry has been
changed.
- The output process is signalled to trigger a response.
Until the garbage-collection timer expires, the route is included in
all updates sent by this router. When the garbage-collection timer
expires, the route is deleted from the routing table.
Should a new route to this network be established while the garbage-
collection timer is running, the new route will replace the one that
is about to be deleted. In this case the garbage-collection timer
must be cleared.
⌨️ 快捷键说明
复制代码Ctrl + C
搜索代码Ctrl + F
全屏模式F11
增大字号Ctrl + =
减小字号Ctrl + -
显示快捷键?